gccrs: ast: Refactor rust-ast-lower-type.h to source file

gccrs: ast: Refactor rust-ast-lower-type.h to source file

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-type.h: Move all remaining definitions to source file.
	* hir/rust-ast-lower-type.cc (ASTLoweringType::translate): Moved to source file.
	(ASTLoweringType::visit): Likewise.
	(ASTLowerGenericParam::translate): Likewise.
	(ASTLowerGenericParam::visit): Likewise.
	(ASTLoweringTypeBounds::translate): Likewise.
	(ASTLoweringTypeBounds::visit): Likewise.
	(ASTLowerWhereClauseItem::translate): Likewise.
	(ASTLowerWhereClauseItem::visit): Likewise.